Shimano Cassette Size Change

Question

I recently purchased a wahoo kickr core and I picked up a Shimano Tiagra CS-HG500 11-32t cassette to put on it. Currently I am running an CS-HG500 11-34t cassette and GRX400 setup. Shimano's website says that these two are compatible and I can use them interchangeably . I noticed once I put the Tiagra cassette on my wahoo I am getting a grinding noise when I pedal and my derailleur won't shift into the smallest gear. What should I do to correct the harsh shifting and grinding that is occurring? Note, I just had the bike serviced and it shifts fine and the chain is in good condition per my local shop.

Answer 1

CS-HG500 is one of the ten-speed cassettes that doesn't come with a 1mm spacer, which is always the first thing to check when significant trainer shifting interchange problems happen with 10-speed. But it doesn't apply here.

The next thing to check is whether the wheel has got a 10-speed cassette installed on an 11-speed freehub body, and if so whether the spacer used is a proper 1.85mm one. If not, it could cause this.

Most of the time, it's an incorrect cassette spacer issue that keeps the interchange from working. Here since the included spacer is on and the cassette doesn't need a spacer, you're more looking at the wheel since there's not a lot that could conceivably be making the trainer incorrect.

Since it's not shifting into the small cog on the trainer, the suspicion is the small cog on the bike is located further in from the dropout. There are a number of possible explanations why, i.e. either a quality or design issue with the hub, something being assembled wrong, a freehub body or other hub part being used as part of a repair that's not a precise match with original spec, etc. If you do enough digging you could probably figure out what the story there is, but the root cause may easily be something you can't do anything about. To fix it, you need to take an accurate measurement of where the first cog starts relative to the frame contact surface on both the trainer and the wheel, and then presuming the wheel is indeed locating the cassette further in, you need to add or reconfigure spacers under the cassette until you've made it match the trainer, and then adjust the rear derailleur accordingly. When you're done you need to check that there's no risk of chain interference with the frame due to the spacer change, which there hopefully won't be because all you're doing is putting it in the standard location. (You can also do this by measuring the distance between the starting shoulder of the freehub body and the frame contact surface. Do whichever works. In this case using the surface of the cog is fine since they're same cassette model.)

Taking the measurement in question can be finicky. On a frame with a large plate dropout it's possible to do it by poking in a vernier caliper, but a lot frames aren't like that anymore. I do it by putting one caliper blade against the outermost surface of the cog and closing one eye to line up the other against the locknut as it floats in space, but you have to be confident at your caliper-squinting abilities for that. You could also clamp something flat and square in with the QR/thru-axle and measure against that.

Note that the potentially confusing part is that the first thing you're doing is making sure an arbitrary spacer configuration hasn't caused the problem, and fixing it if so. But if that's not the problem, you then solve the problem by creating a somewhat arbitrary spacer configuration.

Answer 2

The trainer comes with a 11-speed road freehub body, that is a bit wider than the "classical" Hyperglide freehub body. If I'm not wrong, you have a 10-speed cassette, so you need to install a 1.8mm spacer before installing your cassette. Did you install this spacer?

Note that in general, you may need 2 spacers when mounting a 10-speed cassette on a "regular 11-speed freehub body" (1.8mm + 1mm), but the Wahoo documentation states that only the 1.8mm is necessary: https://support.wahoofitness.com/hc/en-us/articles/204281854-Replacing-the-Cassette